1. Field of the Invention
This invention relates to a process by which plastic parts are formed against an anodically treated aluminum surface by lamination under heat and pressure, whereby the surface of the plastic part, after removal of the aluminum, has a high energy level and is peculiarly receptive to adherent coatings of metal film deposited thereon by standard additive or semi-additive electroless plating processes. The invention finds particular application in the production of electronic circuitboards.
2. Description of the Prior Art
Two distinct methods of manufacture of printed circuitboards for electronic equipment have, in general, been employed in the prior art. One is termed the "subtractive" method, while the other is called the "additive" procedure. The manufacture of a printed circuitboard by the subtractive method starts with a composite consisting of a sheet of insulating (e.g. plastic) material as a base or substrate, having adhered on one or both of its faces a thin copper foil on the order of 0.001 to 0.003 inch thick. This foil is secured to the insulating base by means of an appropriate adhesive and the application of heat and pressure in forming the composite blank circuitboard. The substrate or insulating base used to support the conductive foil is generally made in the form of a flat sheet of compression molded epoxy, phenolic, polyester or other thermosetting resin material, with or without glass fiber or other reinforcement. In preparing a printed circuitboard by the subtractive method, a suitable mask is applied over the foil covered surface or surfaces of the composite to define the desired printed circuit configuration, and the board is then introduced into a spray etching machine or equivalent in order to chemically dissolve away unmasked portions of the copper foil from the surface of the plastic. The resulting circuitboard containing the desired circuit configuration is then treated in various ways to prepare it for mounting of accessory electronic components, etc. The retained portions of the original metal foil thus serve as the conductor paths of the finished circuitboard.
A drawback of the foregoing subtractive method arises from the occurrence during the etching away of the non-circuit areas of a phenomenon known as "undercut" in the metal retaining on the board. Undercut is a term of art employed to describe the lateral undermining of the conductor paths in the resulting circuit pattern on the surface of the board. In fact, this phenomenon of undercutting greatly restricts the fineness or narrowness of the conductor paths that can be tolerated; that is, these conductor paths must be over-designed from a width standpoint to compensate for such undercutting. This of course impedes attempt toward further miniaturization of the circuitboards, an increasingly important consideration in electronic equipment design. Also, where the nature of the printed circuit requires the use of heavier or thicker copper foil on the surface of the plastic substrate, a longer residence time of the board in the etcher must be maintained, during which there is an inherent tendency for the masking material itself to be undermined also, or even partially removed in some areas of the board, thereby causing rejects.
Another major disadvantage of the subtractive method is that the copper-clad board is expensive, and in preparing the printed circuit board it is common for a large portion of the initial copper cladding to be completely etched away. Substantial quantities of etchant solution are utilized in the process, and the solutions must be replaced or rejuvenated periodically to maintain the efficiency of the etching operation. Furthermore, the depleted copper-laden etchant solutions present disposal problems and although they contain large amounts of potentially recoverable copper, the reclamation process involves additional expense so that any savings is of limited economic attractiveness. A further complication arises from the fact that generally the waste etchant solutions cannot simply be discarded without appropriate treatment to avoid pollution, which represents still another expense.
The alternative, i.e. the additive process of printed circuitboard manufacture differs from the subtractive process in that the circuitboards are prepared by directly depositing conductive metal in the desired circuit configuration directly on a suitably prepared plastic substrate. That is, conductor metal is applied to the substrate only where conductor paths are wanted. While this procedure eliminates all (or in the case of a semi-additive process, most) of the etching and waste disposal problems discussed above, it has been subject to problems of obtaining adequate adhesion between the chemically deposited conductive metal and the dielectric substrate. One of the earlier (semi-additive) procedures is described in "Transactions of the Institute of Metal Finishing", 1968, Vol. 46, Pages 194-197. The procedure described involves treating the entire surface of a bare substrate board with a "keying" agent, then plating a very thin initial deposit of electroless nickel over the entire surface to serve as an electrical "bus" for subsequent electroplating. The system then involves the further steps of applying and developing a resist for the desired circuit pattern, followed by additional metal plating by conventional electrodeposition techniques to build up the exposed areas serving as the conductor portions of the finished circuitboards. The resist is then stripped and the printed circuitboard is etched under carefully controlled conditions to remove the exposed regions of the initial metal "bus", leaving only the heavier plate, i.e. the circuit paths, on the finished board. This method has certain advantages, particularly by reason of its semi-additive characteristic in that this facilitates electrodeposition of electrically non-connected circuit patterns on the surface of the board. This enables a desired thickness of the circuit pattern to be built up more quickly than is generally practical with an all-electroless plating operation. However the process described in the article requires especially careful preparation and even application of the keying agent in order to obtain an effective and consistent result. Furthermore, there are problems in finding a suitable keying agent which has dielectric properties compatible with the end-use requirements of the finished circuitboard. And of course the etch-removal of the initial "bus" layer between circuit paths must be done carefully to avoid disintegration of the paths themselves.
Prior to the present invention other procedures for additive plating have included systems of preparing the surface of the resin substrate for electroless plating by a swell-and-etch process involving treatment of the plastic surface with various organic solvents to attack and partially soften the surface, after which the board is subjected to a strong oxidizing solution, for example a sulfuric-chromic acid mixture. This treatment is believed to cause chemical bonding sites to be produced on the surface of the substrate, which sites are sufficiently retained after further extensive rinsing and catalyzing operations to provide a receptive environment for the electrolessly deposited metal film and thereby enhance the adhesion of that film to the substrate. One such method is described in U.S. Pat. No. 3,698,940, granted Oct. 17, 1972. This and similar swell-and-etch systems are subject to the disadvantage of highly toxic and sometimes explosive vapor exposure to operating personnel during the solvent swell treatment, as well as the handling of the highly corrosive etchant materials.
Other additive plating systems have included proposals for incorporating finely divided metal powders or reducible metal salts directly in a surface layer on the plastic substrate in the molding of such substrate, which particles are intended to provide more receptive sites for the reception and retention of the electrolessly deposited conductor metals. Apart from the fact that this requires special molding procedures in the production of the plastic substrates, the dielectric properties of the substrate are adversely affected.